Further samples were taken at East Basin, Ferry Terminal, Lotus Bay and Weston Park East on Thursday and the results were expected on Saturday.

If the levels remain high those zones will be closed for swimming, but will remain open for fishing, canoeing, and rowing.

The authority's chief executive, Gary Rake, said bacteria levels often spiked after storms such as those that swept Canberra on Christmas Eve.

He said the stormwater washed debris, such as duck faeces, into the lake, causing temporary leaps in bacteria numbers. But even if the four areas were closed, Mr Rake said popular swimming spots at Weston Park West, Black Mountain Beach, Central Basin and Yarralumla Beach would be open to bathers.

Water police, based in Yarralumla, patrol the lake regularly checking boat permits.